The present invention relates to a method for producing a NOx removal catalyst for high-temperature exhaust gas, and more specifically, relates to a method for producing a NOx removal catalyst for high-temperature exhaust gas suitable for reductive removal of nitrogen oxides contained in high-temperature gas emitted from fossil-fuel power plants, gas turbines, and the like.
High temperature combustion exhaust gas emitted from fossil-fuel power plants and gas turbines contains nitrogen oxides. The nitrogen oxides in the exhaust gas have to be removed before the emission of the exhaust gas. In this respect, the nitrogen oxides (NO and NO2) are reductively reacted with a reducing agent to perform a decomposition treatment into harmless nitrogen (N2) and water (H2) by providing a NOx removal apparatus downstream of an combustion device, and injecting the reducing agent into the combustion exhaust gas through an injection nozzle. When a method for removing nitrogen oxides from exhaust gas using a NOx removal catalyst is employed here, ammonia (NH3), urea, or the like is added, in general, because it is necessary to cause sufficient NOx removal reaction.
Conventionally, when a NOx removal catalyst is used for reducing the amount of nitrogen oxides in exhaust gas with ammonia being added as the reducing agent, the treatment is carried out in a high temperature region of 300° C. or above, in general. The NOx removal reaction proceeds according to the following formula (1), where the reaction between 1 mol of NO and 1 mol of NH3 results in decomposition into N2 and H2O.4NH3+4NO+O2→4N2+6H2O  (1)
However, in the cases of conventionally used catalysts, oxidation reaction of NH3 itself also proceeds at a high temperature of 450° C. or more according to the following formula (2) or (3), in addition to the reaction of the above-described formula (1).2NH3+5/2O2→2NO+3H2O  (2)4NH3+3O2→2N2+6H2O  (3)
Because of the reaction of the above-described formula (2) or (3), NH3 is not utilized effectively for the reduction of NO, so that the NOx removal performance deteriorates with the elevation of the temperature. Since this causes shortage of the reducing agent, it is difficult to apply the conventional reduction treatment involving the addition of ammonia to, for example, a case of treatment of exhaust gas at 500° C. or above emitted from an outlet of a gas turbine.
Meanwhile, as a method for removing nitrogen oxides at high temperature, a technology using titanium oxide containing a heat resistant inorganic fiber as a support is reported (see Patent Document 1).
However, the catalyst in which tungsten serving as an active component is simply supported on the titanium oxide serving as the support has a limitation in terms of catalytic activity, although the catalyst is optimized based on the mechanical strength of the inorganic filer or the like. Hence, it difficult for the catalyst to efficiently promote the NOx removal reaction.